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General Definition
When an agonist and antagonist muscle contract simultaneously, which occurs when a flexor muscle is active during an extension moment. This can be measured using electromyography (EMG) from the co-contractions that occur between an agonist and antagonist muscle. The general mechanism of it is still not widely understood.

Mechanism of Action

 * 1) The central nervous system sends a signal that travels from the spinal cord to motor neurons
 * 2) Alpha motor neurons stimulate extrafusal muscle fibers (force for muscle contraction)
 * 3) Gamma motor neurons stimulate the ends of intrafusal fibers (length of muscle spindles)
 * 4) Both extrafusal muscle fibers and intrafusal fibers contract during coactivation, constantly sending signals back to the brain about limb position in order to keep the limb at a certain stable position
 * 5) Also known as Alpha-Gamma contraction

Example of Mechanism of Action
Voluntary contraction of the bicep, which is the agonist, leads to the involuntary coactivation of the tricep, which is the antagonist. However, the term agonist and antagonist can be reversed depending upon which muscle is being contracted and relaxed. There is still uncertainty if the mechanism is generalized or localized, since it is situation dependent

How Coactivation is Measured

 * 1) * Muscle co-activation is measured through activity recorded by electromyography (EMG).
 * 2) * The agonist can determined based of the EMG results. Increased EMG activity relates to increased muscle movement, which reveals the role of the agonist and lower EMG activity reveals the antagonist muscle. This concept is very elastic until there are more than two muscles involved in a system. When only two muscles are present in a system, the reversed roles of which muscles is being contracted as the agonist and relaxed as the antagonist can be seen. However, when more muscles are involved in the process, the pattern of activity becomes less obvious to the naked eye.

Factors that Influence Coactivation

 * The weight (load) of the external force applied to the muscle (effects joint stability)
 * The range of the ligaments in the muscle fibers

The Importance of Muscle Coactivation
Abnormal muscle coactivation can be due to damage associated with subcortical and brainstem regions. These regions include the putamen, caudate nucleus, and the thalamus.
 * 1) Allows for joint stability by making the joint stiffer, therefore more stable
 * 2) Allows change in direction of fine movements by protecting ligaments at the end of their range of motion
 * 3) Allows heavier loads to be carried by distributing the force evenly on the muscle fibers
 * 4) Also important in
 * 5) * Locomotion
 * 6) * Isometric activities
 * 7) * General upright standing stabilization